1. Field of the Invention
The present invention relates to a carbon nanotube field emission display, and particularly to an active control applied in a carbon nanotube field emission display.
2. Description of the Related Art
Carbon nanotube field emission display (CNT-FED) is one kind of FED with matrix driving. Each pixel has a cathode, lower plate, with a layer of nanotube for electron emission source and an anode, upper plate, with a transparent electrode for electron attraction. The cathode accelerates electrons to bombard fluorescent material for fluorescence. Arrangements of FEDs of pixels show images. CNT-FED is a high voltage device implemented by two electrodes, anode and cathode, or by three electrodes, anode, cathode, and gate. CNT-FED takes advantage of high voltage and low current to achieve high illumination. In addition to the light and slim properties of LCD (liquid crystal display), CNT-FED has the high illumination properties of CRT (cathode ray tube) displays, making CNT-FED a commercially valuable flat display. The illumination mechanism in three-electrode CNT-FED is briefly described as follows. Fluorescent material is disposed at an anode, and an electron emission source, CNT, with point discharge, is disposed at a cathode. A gate is arranged between anode and cathode to attract electrons from tips of CNTs by high electrical fields and accelerating electrons to bombard the cathode to fluoresce. Therefore, the operating voltage of three-electrode CNT-FED is reduced.
Conventional control of CNT-FED is active or passive control. Passive control is provided by two or three electrodes. FIG. 1 shows a structure of a two-electrode CNT-FED, which has CNT 7, cathode 6, and anode 8. In operating mode, anode 8 provides high voltage inducing field emission at cathode 6, and electrons bombard anode 8 to phosphoresce. In passive control, each pixel is lit instantly by scanning, and duration of providing high voltage at anode 8 is controlled by a PWM (pulse width modulation) signal for adjusting gray level. CNT-FED must provide instantaneous illumination in a short time such that required average illumination is achieved in a frame. The operation of FIG. 1 reduces lifecycle of CNT-FED and requires high control voltages. FIG. 2 shows a structure of three-electrode CNT-FED, which includes CNT 7, cathode 6, anode 8, and gate 9. In normal-on mode, anode 8 provides high voltage to attract electrons from cathode 6, and gate 15 reduces voltage, attracting electrons and achieving low control voltage. Gate 15 also provides a negative voltage to turn off CNT-FED. Gate 15 is utilized to limit the number of electrons to control phosphoresce at anode 8 and controls gray levels. The operation in FIG. 2 still requires PWM signals to control and achieve transient visual, and it also reduces lifecycle.
FIG. 3 shows a structure of active control applied in two-electrode CNT-FED, which includes an active device T1, such as TFT, and a two-electrode CNT-FED, including CNT 7, cathode 6, and anode 8. FIG. 4 shows an equivalent circuit of FIG. 3. In operating mode, a scan line 25 provides voltage to turn on the active device T1, and a signal line 27 is coupled to cathode 6 by the active device T1 to control field emission, inducing phosphoresce, at cathode 6. The scan line 25 provides voltage to turn off the active device T1, thereby extinguishing phosphoresce at anode. Adjustments of gray level are achieved by controlling the duration of illumination. The structure of FIG. 3 has advantages of low control voltage and high anode voltage but still has disadvantages of high instantaneous illumination and short lifecycle.
In order to eliminate disadvantages of high control voltage, high instantaneous illumination, and short lifecycle, there is a need for a combination of an active device, a capacitor, and a three-electrode CNT-FED, which has the advantages of active control, low control voltage, stored control voltage, preserved gray level, high illumination, and reduced power consumption. The CNT-FED of the present invention has a novel control scheme applied in various processes.